1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device with improved picture quality and increased aperture ratio.
2. Description of the Background Art
As the information society develops, displays becomes more important as more information is transmitted via visual media. In order for a display to be of practical use, a display has to have low power consumption and high picture quality while being both thin and light. A liquid crystal display (LCD) not only meets these conditions but also can be mass produced such that new products having a liquid crystal display device can be manufactured rapidly. Thus, the liquid crystal display device is gradually taking the place of the conventional cathode ray tube (CRT) in the visual component industry.
The liquid crystal display device displays an image by using an optical anisotropy of a liquid crystal. Typically, an active matrix (AM) method of driving the display device by an active device, such as a thin film transistor (TFT), is commonly used to control the optical anisotropy of the liquid crystal. More particularly, the liquid crystal display device comprises an upper substrate including a color filter for displaying colors and a black matrix for shielding light, a lower substrate including a pixel area and a thin film transistor used a the active switching device, and the liquid crystal positioned between the upper substrate and the lower substrate.
FIG. 1 is a plan view of a related art liquid crystal display device, which shows an array of pixels on the lower substrate of the liquid crystal display device. As shown in FIG. 1, each pixel is bounded by a gate line 10 and a data line 15. Each pixel area contains a pixel electrode 20 formed adjacent to where the gate line 10 and the data line 15 intersect each other. At the intersection of the gate line 10 and the data line 15, a thin film transistor 30 is positioned.
The thin film transistor 30 comprises a gate electrode 31 connected to the gate line 10, a source electrode 32 connected to the data line 15, a drain electrode 33 connected to the pixel electrode 20. In addition, the thin film transistor includes a gate insulating layer (not shown) for insulating the gate electrode 31 and the source/drain electrodes 32 and 33, and a semiconductor layer 34. A conductive channel is formed in the semiconductor layer 34 between the source electrode 32 and the drain electrode 33 when a gate voltage is supplied to the gate electrode 31.
As shown in FIG. 1, a storage capacitor electrode 40 for maintaining a pixel voltage is arranged in parallel to the gate line 10 in each pixel area. In general, the pixel electrode 20 of the lower substrate, the liquid crystal (not shown), and the common electrode (not shown) of the upper substrate constitutes a liquid crystal capacitor. However, a voltage applied to the liquid crystal capacitor can not be maintained until a next signal is applied because of leakage in the liquid crystal capacitor. Accordingly, in order to maintain the applied voltage on the liquid crystal capacitor, a storage capacitor has to be used with the liquid crystal capacitor to maintain the applied voltage on the liquid crystal capacitor. The storage capacitor not only maintains a signal voltage but also stabilizes gray scale as well as reduces flicker and after-image effect.
There are two methods of forming a storage capacitor. One method is to form the storage capacitor electrode in addition to the other electrodes and then connect the storage electrode to the common electrode. The other method is to use a part of the n−1th gate line as the storage capacitor electrode of the nth pixel area. The former method is called storage on common method or an independent storage capacitor method, and the latter method is called storage on gate method or storage on previous gate method.
The thin film transistor 30, the data line 15, and the storage capacitor electrode 40 shown in FIG. 1 are formed of opaque metal materials that lower an aperture ratio of the pixel area at the time light is transmitted from a back light (not shown) through the lower substrate. The pixel electrode is formed of a transparent conductive material, such as Indium Tin Oxide. To improve the aperture ratio, the pixel electrode is extended over the adjacent data lines 15 and the portion of a black matrix that would be overlapping the pixel electrode along the data line is removed.
FIG. 2 is a plan view showing a part of the liquid crystal display device having a high aperture ratio in accordance with the related art, as discussed above. As shown in FIG. 2, the nth data line 15n and the n+1th data line 15n+1 are located in a row direction, and the gate line 10 is formed in a column direction. A part of the n+1th data line protrudes to form the source electrode 32 of the thin film transistor 30. Also, in the liquid crystal display device having a high aperture ratio, the data lines 15n and 15n+1 are overlapped with a part of the pixel electrode 21 in order to improve the aperture ratio. The cross-hatched areas of S1 and S2 in FIG. 2 denote overlap areas between the data lines 15n/15n+1 and the pixel electrode 21. Because the pixel electrode 21 covers the entire pixel area bounded by the data lines 15n/15n+1, the aperture ratio is increased.
In the related art liquid crystal display device having a high aperture ratio, the data lines 15n and 15n+1 are overlapped with the pixel electrode 21, a thus a parasitic capacitance Cdp is generated between the data lines 15n/15n+1 and the pixel electrode 21. Also, as shown in FIG. 2, the pixel electrode 21 is not formed on the switching device 30, so that an area S1 in which the pixel electrode 21 is overlapped with the nth data line 15n is not equal to an area S2 in which the pixel electrode 21 is overlapped with the n+1th data line 15n+1. Accordingly, a difference is generated between the parasitic capacitances of the right and left sides of the pixel electrode. Such a difference deteriorates the picture quality of the liquid crystal display device. That is, even if the pixel electrode 21 is extended over the data lines 15n and 15n+1 to improve the aperture ratio, a light leakage phenomenon occurs at an edge of the pixel area at the time the pixel is switched from “on” to “off” due to the difference of the parasitic capacitances on the sides of the pixel electrode.